/**
 * Shaders to render 3D volumes using raycasting.
 * The applied techniques are based on similar implementations in the Visvis and Vispy projects.
 * This is not the only approach, therefore it's marked 1.
 */

THREE.VolumeRenderShader1 = {
	uniforms: {
		'u_size': { value: new THREE.Vector3( 1, 1, 1 ) },
		'u_renderstyle': { value: 0 },
		'u_renderthreshold': { value: 0.5 },
		'u_clim': { value: new THREE.Vector2( 1, 1 ) },
		'u_data': { value: null },
		'u_cmdata': { value: null }
	},
	vertexShader: [
		'		varying vec4 v_nearpos;',
		'		varying vec4 v_farpos;',
		'		varying vec3 v_position;',

		'		void main() {',
		// Prepare transforms to map to "camera view". See also:
		// https://threejs.org/docs/#api/renderers/webgl/WebGLProgram
		'				mat4 viewtransformf = modelViewMatrix;',
		'				mat4 viewtransformi = inverse(modelViewMatrix);',

		// Project local vertex coordinate to camera position. Then do a step
		// backward (in cam coords) to the near clipping plane, and project back. Do
		// the same for the far clipping plane. This gives us all the information we
		// need to calculate the ray and truncate it to the viewing cone.
		'				vec4 position4 = vec4(position, 1.0);',
		'				vec4 pos_in_cam = viewtransformf * position4;',

		// Intersection of ray and near clipping plane (z = -1 in clip coords)
		'				pos_in_cam.z = -pos_in_cam.w;',
		'				v_nearpos = viewtransformi * pos_in_cam;',

		// Intersection of ray and far clipping plane (z = +1 in clip coords)
		'				pos_in_cam.z = pos_in_cam.w;',
		'				v_farpos = viewtransformi * pos_in_cam;',

		// Set varyings and output pos
		'				v_position = position;',
		'				gl_Position = projectionMatrix * viewMatrix * modelMatrix * position4;',
		'		}',
	].join( '\n' ),
	fragmentShader: [
		'		precision highp float;',
		'		precision mediump sampler3D;',

		'		uniform vec3 u_size;',
		'		uniform int u_renderstyle;',
		'		uniform float u_renderthreshold;',
		'		uniform vec2 u_clim;',

		'		uniform sampler3D u_data;',
		'		uniform sampler2D u_cmdata;',

		'		varying vec3 v_position;',
		'		varying vec4 v_nearpos;',
		'		varying vec4 v_farpos;',

		// The maximum distance through our rendering volume is sqrt(3).
		'		const int MAX_STEPS = 887;	// 887 for 512^3, 1774 for 1024^3',
		'		const int REFINEMENT_STEPS = 4;',
		'		const float relative_step_size = 1.0;',
		'		const vec4 ambient_color = vec4(0.2, 0.4, 0.2, 1.0);',
		'		const vec4 diffuse_color = vec4(0.8, 0.2, 0.2, 1.0);',
		'		const vec4 specular_color = vec4(1.0, 1.0, 1.0, 1.0);',
		'		const float shininess = 40.0;',

		'		void cast_mip(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray);',
		'		void cast_iso(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray);',

		'		float sample1(vec3 texcoords);',
		'		vec4 apply_colormap(float val);',
		'		vec4 add_lighting(float val, vec3 loc, vec3 step, vec3 view_ray);',


		'		void main() {',
		// Normalize clipping plane info
		'				vec3 farpos = v_farpos.xyz / v_farpos.w;',
		'				vec3 nearpos = v_nearpos.xyz / v_nearpos.w;',

		// Calculate unit vector pointing in the view direction through this fragment.
		'				vec3 view_ray = normalize(nearpos.xyz - farpos.xyz);',

		// Compute the (negative) distance to the front surface or near clipping plane.
		// v_position is the back face of the cuboid, so the initial distance calculated in the dot
		// product below is the distance from near clip plane to the back of the cuboid
		'				float distance = dot(nearpos - v_position, view_ray);',
		'				distance = max(distance, min((-0.5 - v_position.x) / view_ray.x,',
		'																		(u_size.x - 0.5 - v_position.x) / view_ray.x));',
		'				distance = max(distance, min((-0.5 - v_position.y) / view_ray.y,',
		'																		(u_size.y - 0.5 - v_position.y) / view_ray.y));',
		'				distance = max(distance, min((-0.5 - v_position.z) / view_ray.z,',
		'																		(u_size.z - 0.5 - v_position.z) / view_ray.z));',

		// Now we have the starting position on the front surface
		'				vec3 front = v_position + view_ray * distance;',

		// Decide how many steps to take
		'				int nsteps = int(-distance / relative_step_size + 0.5);',
		'				if ( nsteps < 1 )',
		'						discard;',

		// Get starting location and step vector in texture coordinates
		'				vec3 step = ((v_position - front) / u_size) / float(nsteps);',
		'				vec3 start_loc = front / u_size;',

		// For testing: show the number of steps. This helps to establish
		// whether the rays are correctly oriented
		//'gl_FragColor = vec4(0.0, float(nsteps) / 1.0 / u_size.x, 1.0, 1.0);',
		//'return;',

		'				if (u_renderstyle == 0)',
		'						cast_mip(start_loc, step, nsteps, view_ray);',
		'				else if (u_renderstyle == 1)',
		'						cast_iso(start_loc, step, nsteps, view_ray);',

		'				if (gl_FragColor.a < 0.05)',
		'						discard;',
		'		}',


		'		float sample1(vec3 texcoords) {',
		'				/* Sample float value from a 3D texture. Assumes intensity data. */',
		'				return texture(u_data, texcoords.xyz).r;',
		'		}',


		'		vec4 apply_colormap(float val) {',
		'				val = (val - u_clim[0]) / (u_clim[1] - u_clim[0]);',
		'				return texture2D(u_cmdata, vec2(val, 0.5));',
		'		}',


		'		void cast_mip(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray) {',

		'				float max_val = -1e6;',
		'				int max_i = 100;',
		'				vec3 loc = start_loc;',

		// Enter the raycasting loop. In WebGL 1 the loop index cannot be compared with
		// non-constant expression. So we use a hard-coded max, and an additional condition
		// inside the loop.
		'				for (int iter=0; iter<MAX_STEPS; iter++) {',
		'						if (iter >= nsteps)',
		'								break;',
		// Sample from the 3D texture
		'						float val = sample1(loc);',
		// Apply MIP operation
		'						if (val > max_val) {',
		'								max_val = val;',
		'								max_i = iter;',
		'						}',
		// Advance location deeper into the volume
		'						loc += step;',
		'				}',

		// Refine location, gives crispier images
		'				vec3 iloc = start_loc + step * (float(max_i) - 0.5);',
		'				vec3 istep = step / float(REFINEMENT_STEPS);',
		'				for (int i=0; i<REFINEMENT_STEPS; i++) {',
		'						max_val = max(max_val, sample1(iloc));',
		'						iloc += istep;',
		'				}',

		// Resolve final color
		'				gl_FragColor = apply_colormap(max_val);',
		'		}',


		'		void cast_iso(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray) {',

		'				gl_FragColor = vec4(0.0);	// init transparent',
		'				vec4 color3 = vec4(0.0);	// final color',
		'				vec3 dstep = 1.5 / u_size;	// step to sample derivative',
		'				vec3 loc = start_loc;',

		'				float low_threshold = u_renderthreshold - 0.02 * (u_clim[1] - u_clim[0]);',

		// Enter the raycasting loop. In WebGL 1 the loop index cannot be compared with
		// non-constant expression. So we use a hard-coded max, and an additional condition
		// inside the loop.
		'				for (int iter=0; iter<MAX_STEPS; iter++) {',
		'						if (iter >= nsteps)',
		'								break;',

		// Sample from the 3D texture
		'						float val = sample1(loc);',

		'						if (val > low_threshold) {',
		// Take the last interval in smaller steps
		'								vec3 iloc = loc - 0.5 * step;',
		'								vec3 istep = step / float(REFINEMENT_STEPS);',
		'								for (int i=0; i<REFINEMENT_STEPS; i++) {',
		'										val = sample1(iloc);',
		'										if (val > u_renderthreshold) {',
		'												gl_FragColor = add_lighting(val, iloc, dstep, view_ray);',
		'												return;',
		'										}',
		'										iloc += istep;',
		'								}',
		'						}',

		// Advance location deeper into the volume
		'						loc += step;',
		'				}',
		'		}',


		'		vec4 add_lighting(float val, vec3 loc, vec3 step, vec3 view_ray)',
		'		{',
		// Calculate color by incorporating lighting

		// View direction
		'				vec3 V = normalize(view_ray);',

		// calculate normal vector from gradient
		'				vec3 N;',
		'				float val1, val2;',
		'				val1 = sample1(loc + vec3(-step[0], 0.0, 0.0));',
		'				val2 = sample1(loc + vec3(+step[0], 0.0, 0.0));',
		'				N[0] = val1 - val2;',
		'				val = max(max(val1, val2), val);',
		'				val1 = sample1(loc + vec3(0.0, -step[1], 0.0));',
		'				val2 = sample1(loc + vec3(0.0, +step[1], 0.0));',
		'				N[1] = val1 - val2;',
		'				val = max(max(val1, val2), val);',
		'				val1 = sample1(loc + vec3(0.0, 0.0, -step[2]));',
		'				val2 = sample1(loc + vec3(0.0, 0.0, +step[2]));',
		'				N[2] = val1 - val2;',
		'				val = max(max(val1, val2), val);',

		'				float gm = length(N); // gradient magnitude',
		'				N = normalize(N);',

		// Flip normal so it points towards viewer
		'				float Nselect = float(dot(N, V) > 0.0);',
		'				N = (2.0 * Nselect - 1.0) * N;	// ==	Nselect * N - (1.0-Nselect)*N;',

		// Init colors
		'				vec4 ambient_color = vec4(0.0, 0.0, 0.0, 0.0);',
		'				vec4 diffuse_color = vec4(0.0, 0.0, 0.0, 0.0);',
		'				vec4 specular_color = vec4(0.0, 0.0, 0.0, 0.0);',

		// note: could allow multiple lights
		'				for (int i=0; i<1; i++)',
		'				{',
								 // Get light direction (make sure to prevent zero devision)
		'						vec3 L = normalize(view_ray);	//lightDirs[i];',
		'						float lightEnabled = float( length(L) > 0.0 );',
		'						L = normalize(L + (1.0 - lightEnabled));',

		// Calculate lighting properties
		'						float lambertTerm = clamp(dot(N, L), 0.0, 1.0);',
		'						vec3 H = normalize(L+V); // Halfway vector',
		'						float specularTerm = pow(max(dot(H, N), 0.0), shininess);',

		// Calculate mask
		'						float mask1 = lightEnabled;',

		// Calculate colors
		'						ambient_color +=	mask1 * ambient_color;	// * gl_LightSource[i].ambient;',
		'						diffuse_color +=	mask1 * lambertTerm;',
		'						specular_color += mask1 * specularTerm * specular_color;',
		'				}',

		// Calculate final color by componing different components
		'				vec4 final_color;',
		'				vec4 color = apply_colormap(val);',
		'				final_color = color * (ambient_color + diffuse_color) + specular_color;',
		'				final_color.a = color.a;',
		'				return final_color;',
		'		}',
	].join( '\n' )
};
